I'm not Ace, but nightly builds are more recent than the RC builds. They have some more fixes and features that haven't been finished in time for 0.45.0, and though the difference is just a week or so of the commit history of the repository they are likely to get far ahead of the release builds when 0.45.0 is released.

The difference of course is that nightly builds are built off of the latest commit in master at the point of compilation, while RC builds are built from the latest commit in the release branch. (openmw-45 in this case)

Nightlies are great for trying out new features, while RCs are invaluable for making sure that the game will run properly as a release.